Terrible, of course. Especially since they are aiming the service to improve sign-up reliability in countries that block telegram, acting as a relay exposes yourself. Carriers in China (where I live) and other questionable countries are actively snooping around, and since SMS are generally unencrypted, the simplest heuristic would figure out what you're involved in and start a very serious investigation.
On top of that, phone numbers in many countries are also unique logins to a number of services (again, here in China you need it for literally everything, it's THE number one digital footprint), and attackers could use the information for bruteforce/wordlist attacks on known services, or use them for social engineering.
As much as I like the idea of helping others sign up who don't have the means to acquire a foreign phone number, I would never willingly commit to that.