In this paper, we present a novel joint signature protocol suited to applications run on UMTS and heterogeneous networks. The protocol enables a mobile user to securely delegate his/her signing power to an assisted server so that the assisted server can perform signature generation and verification on behalf of the user. By shifting computational expensive cryptographic operations to a third party, i.e. the assisted server, the protocol is able to eliminate public key operations for the mobile end, while providing message origin authentication, integrity, and non-repudiation services to both delegated (i.e. the mobile user) and delegating (i.e. the third party) entities. The protocol is suited to m-commerce applications, in which a mobile user with a resource restrictive device wishes to access Internet-based security-sensitive services. The security of the proposed protocol is analyzed and its performance against other related work is evaluated.