Any computer could make a phone call; having a handler in place for "tel:" is certainly the first step (but not even really necessary!) No amount of probing whether this is a mobile device, or has a SIM card, or has properties of Android or whatever, is going to generously include all devices that are capable of placing PSTN calls.
I mean, yeah, check and see whether it has a microphone and audio output - that's fairly mandatory for telephony operations. Other than that... dunno.
Microphone is not strictly required for telephony. After all, if you're willing to add a few processing steps, (recording and file transfer), even a near audioless computer can handle marshalling a telephony message.
Granted, yes, but I think that most developers who ask "can this device make a phone call?" are actually asking the question whether the human operator of the device can have a conversation over the telephone, so yeah.
I mean, yeah, check and see whether it has a microphone and audio output - that's fairly mandatory for telephony operations. Other than that... dunno.