.TEL ... is it a perspective domain which sibername company offers?
What is the pricing of it?
Are they ICANN-accredited?

Yes, sibername is accredited by ICANN, you can also register .tel domain name at Domain name registration there price is one of the most competitive in UK and price is straight forward with not set-up fee, no release fee when moving your domain and not gimmicky pricing that makes prices appears cheap.

Want to add that sibername isn't only ICANN but CIRA and EURID accredited.
That's a reliable company with affordable rates and quality support.