Whether or not a physical therapy assistant (PTA) may perform physical therapy modalities depends on two factors: state law and payer policies. 

First, verify with your state to see which modalities are allowed under state scope of practice laws.

Second, check the individual payer policy because even if it is allowed under state law, it might not be allowed by the payer. For example, Medicare only allows licensed therapist assistants (this does NOT include physical therapy technicians) to provide therapy services in an outpatient private practice setting ONLY if those services are performed under the direct supervision of a licensed therapist. See 42 C.F.R. §410.26(b)(1)-(7) and CMS Medicare Benefit Policy Manual, Pub. 100-4, Ch. 15, §60.1 - §60.5. However, there are a list of conditions which must be met such as the supervised therapist performs the evaluations and establishes a plan of care and the supervising therapist must be on-site and sign the documentation.