What is the ruling on accepting money from my mother from the sale of land that she claims is hers, given a previous dispute over it with relatives, and is it permissible for me to contact my brother via a phone he bought with this money before verifying the lawfulness of this money?

There is no objection to accepting a gift from the mother, even if it is from money obtained from the sale of a disputed land, or if it is from her private funds. The basic principle is that ownership remains hers until proven otherwise by Sharia. The rights of others pertain to her liability, not yours. There is also no objection to using a phone purchased with the money from the land sale. Considering this as a means to block illicit avenues (sadd al-dhari'ah) has no basis, because an expansive application of blocking illicit avenues leads to undue strictness and difficulty in religion.