Ace's phone is a MV-TAP. It allows communciation between different universes beyond the Undertale worlds. Numbers are added to the it by pressing the back of the MV-TAP to the phone. It is surrounded by a protective case that allows the phone to survive pretty much any blow, extremely high and low temperatures and being submerged. The back of Ace's phone has the Citadel's logos (a fox head with wings and nine tails), the MV-TAP tends to have the logo of the owner's homeworld on the back. It has a touchscreen that works with gloves. The passcode on the MV-TAP is a five dimensional puzzle.
Obtaining a TAP
MV-TAPs are only accessible to Solos, beings that there are only one of in the entire multiverse (not just the Undertale section, the mulitverse as a whole). Trying to use a TAP while being a non-solo can be fatal and has been known to drive people insane so is not adviced. You must be introduced to the company by another Solo to gain one.
The WORLD alternative is much safer and easier to get and is done in the same method as the MV-BANK. Draw the attention of the company by travelling to other universes and they will leave a card for you in your home base with instructions.
The phones numbers in MV-TAP are absurdly long and impossible to remember. The first set of numbers relate to the dimensions the creatures exist in. The second is set whether or not they travel forward or backward in time and if they travel linearly. Next is the world type. The forth set relates to the reality bubble the world is in. Then the secondry world type(and tertiary world if it's an AU of an AU). The sixth is the indivdual world and finally the indivdual.
For example, if he would try to call Akin!Sans the number would be for the code for:
1st, 2nd, 3rd dimensions, forward linear time travel, game world, Undertale, web comic world, Akintale and Sans.
This code is written in Hexadecimal to shorten it a little, The code for Akintale alone is 16 digits long. (416b696e74616c65).
What it can access
The MV-TAP abstracts information from any version of the internet. This means that the TAP has access to websites like DA, Tumblr, the UTWikia/UTAUWikia and Youtube of any unverse including our own. The TAP can only be used by Solos (beings that there is only one of, there is no alternative version of them).
Lesser beings can have an MV-WORLD (a MultiVerse Worldwide Observation Reality Link Device) which also allows for communication between worlds but is limited to the reality bubble. Numbers in the WORLD is much shorter as it doesn't have to have access to the first 3 sets of codes. It can be extended out to those sections if they encounter somone else this an MV-WORLD within their bubble. The device also allows for the user to hack into any kind of camera within the buuble but an only be actively connected to five at any one time, it does however include a tracker that can be found using a TAP and the WORLD can be disabled by the TAP user at any moment.