You don't have a defense. The statute prohibits holding a phone other than what it is necessary to instigate or terminate a hands free call.
Below is the law but I don't see anything about holding a device only using it.
§21–1124.2.
(a) (1) In this section the following words have the meanings indicated.
(2) “Handheld telephone” means a handheld device used to access wireless telephone service.
(3) “9–1–1 system” has the meaning stated in § 1–301 of the Public Safety Article.
(b) This section does not apply to:
(1) Emergency use of a handheld telephone, including calls to:
(i) A 9–1–1 system;
(ii) A hospital;
(iii) An ambulance service provider;
(iv) A fire department;
(v) A law enforcement agency; or
(vi) A first aid squad;
(2) Use of a handheld telephone by the following individuals when acting within the scope of official duty:
(i) Law enforcement personnel; and
(ii) Emergency personnel;
(3) Use of a handheld telephone as a text messaging device as defined in § 21–1124.1 of this subtitle; and
(4) Use of a handheld telephone as a communication device utilizing push–to–talk technology by an individual operating a commercial motor vehicle, as defined in 49 C.F.R. Part 390.5 of the Federal Motor Carrier Safety Regulations.
(c) The following individuals may not use a handheld telephone while operating a motor vehicle:
(1) A driver of a Class H (school) vehicle that is carrying passengers and in motion; and
(2) A holder of a learner’s instructional permit or a provisional driver’s license who is 18 years of age or older.
(d) (1) This subsection does not apply to an individual specified in subsection (c) of this section.
(2) A driver of a motor vehicle that is in motion may not use the driver’s hands to use a handheld telephone other than to initiate or terminate a wireless telephone call or to turn on or turn off the handheld telephone.
(e) (1) A person convicted of a violation of this section is subject to the following penalties:
(i) For a first offense, a fine of not more than $75;
(ii) For a second offense, a fine of not more than $125; and
(iii) For a third or subsequent offense, a fine of not more than $175.
(2) Points may not be assessed against the individual under § 16–402 of this article unless the offense contributes to an accident.
(f) The court may waive a penalty under subsection (e) of this section for a person who:
(1) Is convicted of a first offense under this section; and
(2) Provides proof that the person has acquired a hands–free accessory, an attachment or add–on, a built–in feature, or an addition for the person’s handheld telephone that will allow the person to operate a motor vehicle in accordance with this section.
