As long as you take precautions the use of a phone number for telegram is not really a big issue. Go take the dog for a walk over the banks or something then turn on ure phone, create telegram acct, then sign in on a tablet and throw the simcard. Done. Well you get the idea.
Session is built on Lokinet, just is just a giant mesh / extra layer that routes all of it's traffic through the Onion network. So if you feel comfortable, there's no reason you shouldn't feel comfortable there.
we use session but can b a bit buggy at time with dupe msg'ing
but if u want easy session is it, your setup within sec's of running it, but if you are sending anything sensitive data (tracking/wallet/postal addy) on any app pls use pgp... never trust any app or service to protect your data imo
also on session u can use ONS's to contact people who have them which is much easier than the long number user id